This invention relates to laser beam addressed smectic displays and their manner of operation, and is particularly concerned with displays of this type that are suited for direct projection whereby lines written into the display by the laser beam project as bright lines set against a dark background.
If a homeotropically surface aligned liquid crystal cell containing a filling that exists in a smectic phase exhibiting positive dielectric anisotropy is, in the absence of any applied electric field, slowly cooled from a temperature at which the filling exists in the isotropic liquid phase to a temperature at which it exists in the smectic phase, then the visual clarity of the liquid in the isotropic phase is preserved when it enters the smectic phase. If however, the cooling is sufficiently rapid it is found that the rapid quenching of the random orientation of the molecules in the isotropic phase gives rise to the formation of focal-conic domains in the smectic phase, and those domains are light scattering.
This effect can be used to create a laser beam written display in which the whole cell is subjected to a relatively slow thermal excursion into the isotropic phase and back into the smectic phase in order to erase the cell, rendering it clear so as to be ready for the entry of data. The data is entered by tracking a laser beam over the surface of the display which is arranged to absorb the incident light and thus produce sharply localised heating. Movement of the beam across the surface is arranged to be sufficiently rapid to produce the rapid cooling necessary for the production of focal-conic scattering domains. In this way light-scattering tracks are written which contrast with a clear background. A conventional projection system will project these as dark lines set aqainst a bright background. This is the wrong way round for an overlay projection system in which the projection system is to be used to add the information content of the liquid crystal display as an overlay to an existing display. For an overlay projection system the information needs to be projected in the form of bright lines set against a dark background. Reverse Schlieren optics can be used to convert the scattering tracks into this format, but such an optical system is very wasteful of light.
Clear tracks can be written into a field which has previously been set into a scattering state by traversing the laser beam whilst maintaining an alternating electric potential across the thickness of the layer. The frequency of this alternating potential needs to be above a certain threshold value beneath which ion migration effects will tend to reinforce scattering rather than extinguish it by inducing homeotropic alignment. The magnitude of the resulting field must be somewhat less than the threshold value which will restore homeotropic alignment in the absence of the thermal in pulse provided by the laser beam.
The practical implementation of this method of laser beam writing of clear tracks in a scattering field requires a satisfactory method of setting the field into the scattering state in the first instance. In principle this scattering state could be produced by scanning the laser beam over the whole display area in the absence of any applied field, but this is too time consuming to be practical for most applications.
An alternative method, which is the subject of our Patent Specification No. 2093206A, is to illuminate the whole display area with a short duration flash of intense light, for instance from a xenon flash, which is absorbed and thus produces the requisite short duration heating pulse. One of the problems with this approach is the difficulties encountered in obtaining acceptable uniformity illumination, and hence of scattering density.
The present invention is concerned with an alternative approach which involves modifying the filling of the cell by the incorporation of a dopant that provides it with an anisotropic conductivity suitable for inducing dynamic scattering within the liquid crystal layer so that it can be set into a scattering state by the application of an electric field.